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Math Help - dot product

  1. #1
    Junior Member
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    Jul 2008
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    27

    Cool dot product

    Think im being stupid with this one but ill ask in anycase:

    I have a descent down a hill measured at -2. I need to find the angle of the descent so i use the following formula:

    /a//b/cos(theta)

    so rearranging :

    arccos a . b / /a//b/

    where a = -2 and b is the unit vector i. Right when Iam working this out i get pi which is ridiculous as this means the descent is vertical!!! clearly im working something out wrong but not sure what it is.


    (Note : I am learning this stuff in my spare time - i am firefighter full time - so if this seems really basic to some i do apolgise)
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  2. #2
    Newbie
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    Jul 2008
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    Calculate arccos\left(\frac{[-2,1]\cdot[1,0]}{\sqrt{5}}\right). It should give you the angle. Remember that the cosine is symmetrical along the x-axis (in the unit circle).
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  3. #3
    Junior Member
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    Jul 2008
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    153.4 degrees or 2.67 rad s-1 ????
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